A conventional resistive circuit can be achieved by connecting several resistors with fixed resistances in parallel. The total equivalent resistance of these resistors can be adjusted by controlling the on/off states of a plurality of switches which are connected between the parallel resistors. Therefore, the equivalent resistance of the resistive circuit can be varied. The resistive circuit is usually used in a switchable RC filter of DC offset cancellation circuit to provide the variable resistance. However, the variable total resistance of the parallel resistors of the resistive circuit is not varied in a continuous form but in a discrete form. The discrete variable resistance of the switchable RC filter will cause a poor frequency response and a longer transient time. Therefore, the switchable RC filter with a discrete variable resistance cannot reliably eliminate DC offsets.
Hence, there is a need to provide a reliably controllable resistive circuit capable of providing a variable resistance which is controlled to vary continuously and smoothly.